Output stages are part of the amplifier circuits and are counted among the most important basic circuits in analog circuitry.
Output stages can be distinguished further using various characteristics, for example, by their small-signal gain response. An exemplary embodiment of a normal-mode AB output stage is shown in prior art FIG. 5. The output stage shown comprises two series-connected field effect transistors of different conductivity type. Specifically, one connection of a p-channel field effect transistor 900 is connected to a supply potential VDD and its other connection is connected to the output 905. Connected in series with this is an n-channel field effect transistor 901. The two field effect transistors 900 and 901 are actuated in antiphase by a normal-mode signal VIN at the signal connection 902.
To set the operating point, there is additionally provided a voltage source 904 which is connected to the control connections of the two transistors for the purpose of setting the operating point of the normal-mode AB output stage. In this case, the operating point is chosen such that over the entire input and output voltage range there is a gain in a respective one of the two field effect transistors.
FIG. 6 shows the dependency of the two currents I1 and I2 flowing through the transistors on the input voltage VIN. To reduce distortions in small-signal operation around the operating point V0, the diodes are used to set the quiescent output current IQ such that it is always greater than zero. A drawback is the great dependency of the output current on the supply voltage and external parameters which cannot be influenced. These include the temperature, for example. If the voltage supply VDDfluctuates and is not constant, these are transferred directly to the response of the circuit shown in prior art FIG. 5. The problem also arises if the voltage for setting the operating point is derived from the supply voltage. There is no provision for the output current Iout to be limited in the class AB output stage shown in FIG. 5.